1. Field of the Invention
This invention relates to a signal transmitting method and a communication system suitable for use in a system in which a plurality of mobile stations can access a common access channel, for example.
2. Discussion of the Prior Art
In a digital signal transmission system in which a plurality of terminals can access a common channel, it is necessary to use identification data to identify a transmitter terminal and its communication counterpart for the following reason. If the transmitter and receiver terminals are unknown, the signal itself becomes impossible to transmit because the common channel can be accessed by many terminals. The same is true when a packet communication is performed in a mobile telephone.
Under the above-mentioned circumstance, it is customarily known that a certain signal is to be transmitted from a known transmitter terminal to a known receiver terminal. Therefore, identification data indicative of the receiver terminal (or mobile station) can easily be intercepted or monitored. For this reason, intentional interference can easily be performed when so desired. For example, some kind of signal can readily be sent to the counterpart terminal for the purpose of interference.
As a preventative measure, a signal can be made secret or concealed. For the sake of convenience in the communication system, however, it is practically impossible to make secret or conceal data of the receiver station and therefore, it is only data of the transmitting signal that can be made secret.
FIG. 16 illustrates one example of a construction for enciphering a signal having a transmitting data generator 1 for generating data to be transmitted. Data DA prepared in the transmitting data generator 1 is enciphered by an encipherment processor 2 in accordance with a given algorithm. For encipherment, a key Key is used. For decoding the enciphered signal, the receiver may use the same key as the transmitter side.
In that case, since the transmitter and the receiver are asynchronous, the receiver cannot appropriately change the key Key but is required to use a predetermined fixed key.
In the exemplary construction in FIG. 16, the signal can be made secret if so desired. However, when the data are simple, the original data can, in some cases, be guessed simply by seeing the enciphered result. Therefore, the exemplary construction does not have sufficient concealability to prevent interference.
In the case of control signals for a mobile communication, for example, the data are limited and the kinds of data are also small, only three in some cases. On the other hand, if the signal is enciphered using a fixed key in accordance with the same algorithm, the original signal can easily be derived because the kind of enciphered signal becomes the same in number as the original signal. That is, with the method for enciphering a signal using the same key, when the kind of patterns of the signal is small, the pattern of the enciphered result also becomes small. Thus, the original signal can easily be derived.
For the reasons mentioned above, when a signal should be monitored by a third party who has a good knowledge of the system, there is a large possibility that the transmitted or received signal can be guessed as to what the signal is, based on data obtained from the monitored result, such as the frequency of occurrence of a certain pattern, signal length, etc., which are the results of encipherment. Especially, when the signal in question is a control signal, the purpose of the control signal can be guessed (for example, connection, disconnection, and the like).
Once a third party knows that a signal is transmitted for a given purpose, the third party can generate a signal identical to the one which the genuine terminal generates without knowing the key for concealment. As a consequence, it becomes possible for the third party to interfere with the communication pretending to be the genuine terminal.
In case, for example, of a packet communication using a mobile telephone in which the number of different control signals is small, a simple encipherment is not a sufficient preventive measure of possible interference for the reasons mentioned above. In addition, since a signal is transmitted in a communication system which is easily monitored, an effective preventive measure against possible interference is required.